A nonmetallic plasmonic catalyst for photothermal CO2 flow conversion with high activity, selectivity and durability

The meticulous design of active sites and light absorbers holds the key to the development of high-performance photothermal catalysts for CO2 hydrogenation. Here, we report a nonmetallic plasmonic catalyst of Mo2N/MoO2-x nanosheets by integrating a localized surface plasmon resonance effect with two distinct types of active sites for CO2 hydrogenation. Leveraging the synergism of dual active sites, H2 and CO2 molecules can be simultaneously adsorbed and activated on N atom and O vacancy, respectively. Meanwhile, the plasmonic effect of this noble-metal-free catalyst signifies its promising ability to convert photon energy into localized heat. Consequently, Mo2N/MoO2-x nanosheets exhibit remarkable photothermal catalytic performance in reverse water-gas shift reaction. Under continuous full-spectrum light irradiation (3 W·cm−2) for a duration of 168 h, the nanosheets achieve a CO yield rate of 355 mmol·gcat−1·h−1 in a flow reactor with a selectivity exceeding 99%. This work offers valuable insights into the precise design of noble-metal-free active sites and the development of plasmonic catalysts for reducing carbon footprints.

Reviewer #2 (Remarks to the Author): In this manuscript, Wan et al. have designed an efficient photothermal catalyst of Mo2N/MoO2-x, which exhibits a combinafion of high acfivity and stability to catalyze the RWGS reacfion under relafive mild reacfion condifions.The presence of localized surface plasmon resonance (LSPR) effect efficiently converts photon energy into localized heat, thus promofing the acfivafion of H2 and CO2 at N atoms and O vacancies, respecfively, on the catalyst surface.This work can be regarded as a good reference to guide advanced catalysts in photothermal catalysis by ufilizing the LSPR effect.However, although various insitu characterizafion techniques have been measured to invesfigate the structure-funcfion relafionship, the intrinsic structure of the catalyst and the origin of the LSPR effect are ambiguous, which largely limits the innovafion and impact of this work.In summary, this manuscript needs major revisions.The specific comments are as follows: 1. Specific sizes of Mo2N and MoO2 are not available from HRTEM images.Please provide informafion about the specific size of Mo2N and MoO2. 2. Compared to the standard card, why the XRD peaks of Mo2N and MoO2 exhibit obvious shift?Please explain it.3. Mo2N can be easily oxidized by O2.Is the surface structure of the synthesized Mo2N oxidized by O2 in air?From the element distribufion in Figure 1d, how to understand that the spafial distribufions of Mo, N and O are highly correlated? 4. The authors menfion that the composifion rafio of MoO2-x and Mo2N can be tuned by controlling the annealing temperature.Is it possible to quanfify the composifion rafio of Mo2N and MoO2 accurately? 5. Compared to other noble metal-free catalysts, why this Mo2N/MoOx catalyst can generate the strong LSPR effect?And how to understand the interacfion between Mo2N and MoOx?It seems that there is no detailed discussion in this manuscript to explain the origin of the LSPR effect over Mo2N/MoOx.6.The authors emphasize that the synergy between Mo2N and MoO2-x is crucial to opfimize the RWGS reacfion acfivity.However, it seems that Mo2N and MoOx just serve as separate sites to acfivate H2 and CO2, respecfively.7.Although the oxygen vacancy concentrafions of 550,and 650 are very different, their ability to adsorb CO2 is similar (Supplementary Fig. 9).Therefore, the relafionship of oxygen vacancies and CO2 adsorpfion is ambiguous in this manuscript.8. Generally, metal sites have a strong capacity to dissociate H2 molecule.For Mo2N/MoOx, how to understand the role of exposed Mo sites in dissociafing H2?
Reviewer #3 (Remarks to the Author): Liu, Xiong and coworkers report the preparafion of Mo2N/MoO2-x nanosheets as catalysts for the lightpowered RWGS reacfion.The preparafion and in-depth structural and composifional characterizafion of the catalyst materials produced in this study are reported very well.However, with the generafion and interpretafion of the obtained catalysis results I have some concerns that need to be addressed before considering publicafion (see below).If these concerns are addressed properly, this study could be highly significant and impact the field of photothermal plasmon catalysis.

Concerns:
-For noble metal/metal oxide combinafions, you report systems with rather low acfivity (lines 61-67).One of the higher acfivifies reported to date are obtained for Au/TiO2.See e.g.Plasmon-enhanced reverse water gas shift reacfion over oxide supported Au catalysts -Catalysis Science & Technology (RSC Publishing) and Low Temperature Sunlight-Powered Reducfion of CO2 to CO Using a Plasmonic Au/TiO2 Nanocatalyst -Marfínez Molina -2021 -ChemCatChem -Wiley Online Library.
-You claim that it is necessary for the applicafion of photothermal catalysis to move away from the use of noble metals (lines 72-75).Please note: the noble metal does not always dominate the levelized costs of the product.Recently, for photothermal CO2 methanafion it was demonstrated that the levelized cost of green H2 was dominafing the CH4 cost price (Renewable natural gas as climate-neutral energy carrier?-ScienceDirect).
-You tested the long term stability (160 h) of you catalyst under steady state operafing condifions (fig. 2d).Normally, one would expect a slight decrease of the performance over fime due to water adsorpfion.You don't seem to see that.Why is that the case?What are the catalyst bed temperature and reactor temperature?-You tested the long term stability (160 h) of your catalyst under steady state operafing condifions (fig. 2d).For sunlight powered chemical processes, stability of your catalyst during repefifive on/off experiments is also of importance.Did you test that?-Figure 2f implies things that are not correct.It is misleading.You cannot simply compare the acfivity of your catalyst 1-to-1 to other reported catalysts, unless the test condifions are idenfical (e.g., same irradiance, same feedstock rafio, same flow, etc.).You results are e.g.generated with double the irradiance of the results reported by Molina et al. (see above).Furthermore, the fact that other groups report shorter stability may also simply be aftributed to the fact that they didn't test it longer that a couple of hours.
-Are you sure the broadband absorpfion of your catalysts is based on the LSPR effect?I know that plasmonic resonances are broad, but they typically do lead to a rather disfinct absorpfion maximum.
-What is the highest apparent quantum efficiency you achieve (I think you call it ITC in the Supplementary Info)?This seems rather low compared to state of art systems which is not in favor of a substanfial non-thermal contributor.
-How did you determine the surface temperature of your catalyst?Insufficiently careful measurements can lead to rather large underesfimafions (see e.g., Using Fiber Bragg Grafing Sensors to Quanfify Temperature Non-Uniformifies in Plasmonic Catalyst Beds under Illuminafion -Xu -2022 -ChemPhotoChem -Wiley Online Library).Accurate temperature measurements, incl.temperature gradient measurements, are needed to substanfiate your claim on a non-thermal contributor (lines 179-182).Also other types of experiments, such as a study of the acfivity as funcfion of irradiance, may contribute (see e.g., hftps://www.nature.com/arficles/s41377-020-00345-0).

Reply to Reviewers' comments
Reviewer #1 (Remarks to the Author): This manuscript presents the utilization of Mo2N/MoO2-x nanosheets for the photothermal catalytic reverse water-gas shift (RWGS) reaction.During an extended 168-hour experiment under continuous full-spectrum light irradiation (3 W•cm -2 ), these nanosheets demonstrated a CO yield rate of 355 mmol•gcat -1 •h -1 with a selectivity of 99%.However, it is worth noting that this study can be viewed as an extension of recent research published in Nature Communications (2023,14,2551), which regrettably was not referenced by the authors.A comparative analysis reveals that the Ni3N nanosheets featured in the aforementioned Nature Communications paper exhibited a catalytic activity that was at least four times greater than that of the Mo2N/MoO2-x nanosheets presented here.Consequently, the primary distinction in this work is just the substitution of Ni3N nanosheets with Mo2N nanosheets, albeit with poor activity.
Reply: We really appreciate the reviewer's insightful suggestions to help us significantly improve the quality of our manuscript.We have carefully revised the manuscript and sincerely hope that our revisions have satisfactorily addressed the reviewer's concerns.Firstly, we are very sorry for neglecting the work of Ni3N, which investigated the role of plasmon excitation.It can also prove the importance of developing efficient nonmetallic plasmonic catalysts for photothermal catalytic applications.However, the excellent catalytic activity of Ni3N (1212 mmol•gcat -1 •h -1 ) only appeared once in the article and cannot be maintained for a long time.As we all know, besides catalytic activity and selectivity, the stability and conversion rate are also regarded as the evaluation criteria for catalyst performance.In long-term stability study, the catalytic activity (325 mmol•gcat -1 •h -1 ) of Ni3N decreased by ~30% within 25 hours under 2.5 W•cm -2 , while the catalytic activity of Mo2N in our work can maintain for 190 hours even under a higher light intensity (3 W•cm -2 , 355 mmol•gcat - can reach up to 1345 mmol•gcat -1 •h -1 for the first 5 minutes in a batch reactor, which is consistent with the reaction equilibrium time in sealing system (Fig. R1/Supplementary Fig. 5).Moreover, there is also a significant difference between our catalyst and Ni3N on the catalyst design concept.The synergistic effect of dual active sites in our study will provide a convenient avenue for the design of efficient photothermal catalysts.
Above all, we remain convinced that our work is worth reconsidered by Nature Communications.
Table R1.The comparison of catalytic activities with Mo2N/MoO2-x and Ni3N for photothermal catalytic CO2 hydrogenation.comprehensive examinations into intensity dependence, kinetic isotope effects (KIE), quantum efficiency, and related aspects.Furthermore, the claim regarding the "photothermal" (Vs"non-thermal") lacks substantiated proof and experimental support.
Overall, I see this MS as routine work and lacks novelty as well as in-depth mechanistic studies to be published in Nature Communication.
Reply: We appreciate the concern and constructive comments raised by the reviewer; however, the reviewer may have overlooked the novelty demonstrated in this work.Our point-by-point responses are listed below: 1. We have explored the surface temperatures and CO generation rates of MNO-550 under different light intensities as shown in Supplementary Fig. 12b, and both can be enhanced with increasing light intensity.The remarkable distinction of surface temperature indicates that the Mo2N/MoO2-x nanosheets can efficiently promote charge transfer and accumulation of abundant plasmon hot electrons on the catalyst for thermal energy generation, which presents an excellent capacity for photothermal conversion.4. To explore the "photothermal" Vs "plasmonic non-thermal" effects, we have evaluated the catalytic performance of MNO-550 in successive light on and off conditions (Fig. R4/Supplementary Fig. 10).The CO generation rate decreases sharply when the light is turned off, which is similar to the performance of Ni3N (Nat. Commun. 2023, 14, 2551).Due to slower response rate of thermal energy compared to light energy, the ambient temperature will be maintained for a while after turning off light.We find that after switching the light off, the catalyst will not show any obvious activity in our system.Obviously, the LSPR effect has made a significant contribution.
It is worth noting that the thermal effect is also crucial for driving the RWGS reaction.In fact, it is not easy to eliminate the influence of thermal effects in a photocatalysis or photothermal system.As shown in Supplementary Fig. 12c, both the surface temperature of the sample and the generation rate of CO decrease with ice bathing.However, the performance is still better than that of thermal catalysis even at 250 ℃ due to the ability of photothermal conversion in Mo2N/MoO2-x.
Overall, Mo2N/MoO2-x as a novel nonmetallic plasmonic catalyst with dual active sites exhibits its superior photothermal RWGS performance Reply: We greatly appreciate the reviewer's positive and valuable comments.We have carefully revised the manuscript and sincerely hope that our revisions have satisfactorily addressed the reviewer's concerns.
1. Specific sizes of Mo2N and MoO2 are not available from HRTEM images.Please provide information about the specific size of Mo2N and MoO2.
Reply: We appreciate the reviewer's valuable comments.Our samples are mainly porous nanosheets with high specific surface area and uniformly distributed elements of Mo, N and O. Therefore, conventional morphology characterization is difficult to measure the specific size of catalyst particles accurately.In order to acquire the specific size, we have complemented high-resolution TEM characterization (Supplementary Fig. 1a and Fig. R5).Based on the HRTEM image below, we can observe that the porous nanosheets consist of well-distributed Mo2N and MoO2 area with 2~5 nm size.In addition, the observed shift of peak centered at ~37° is mainly caused by the variation of intensity ratio of two overlapping peaks, i.e., 37° attributed to MoO2 (-211) crystal plane and 37.7° attributed to Mo2N (112) crystal plane.When increasing the ammoniating temperature, the intensity of MoO2 (-211) peak decreases but the intensity of Mo2N (112) peak increases.As one falls another rises, the overlapped peak exhibits a slight shift toward a higher diffraction angle.

Mo2N can be easily oxidized by O2. Is the surface structure of the synthesized Mo2N
oxidized by O2 in air?From the element distribution in Fig. 1d, how to understand that the spatial distributions of Mo, N and O are highly correlated?
Reply: We appreciate the questions raised by the reviewer.After ammoniation of MoO3 precursor, the as-prepared sample will quickly be partially oxidized upon removal from the tube furnace and exposure to air.Similar phenomenon has been reported by Kim et al. (Ref. 25 Appl. Surf. Sci. 1999, 152, 35-43).For the highly correlated spatial distribution of Mo, N and O shown in the Fig. 1d, we try to understand from two aspects.
On one hand, during the catalyst synthesis process, the porous nanosheets with high specific surface area facilitate full contact between the surface of MoO3 precursor and NH3 reactant.As a result, the distribution of N and O in the catalyst is relatively uniform.

Reply:
We appreciate the reviewer's valuable comments.The LSPR effect in Mo2N/MoO2-x nanosheets occurs from the collective oscillation of conduction electrons at the surface of the nanoparticles under the excitation of incident light.The interaction of LSPR between Mo2N and MoO2-x is related to the microstructure of the catalyst, which has been replied in the comments above.There is a large number of interfaces between Mo2N and MoO2-x components in Mo2N/MoO2-x nanosheets and the interfacial interaction can regulate the strength of LSPR effect.Both Mo2N and MoO2 components are reported as nonmetallic plasmonic materials in the previous literatures (Nano Lett. 2021, 21, 4410-4414;Appl. Catal. B 2022, 319, 121887).Mo2N component can enhance the absorbance of catalyst in the vis-NIR region (Fig. 2a), while the oxygen vacancies in MoO2-x can capture the photogenerated electrons to promote charge separation.The existence of interfaces can enhance photogenerated charge transfer and the accumulation of hot carriers can increase the surface temperature of catalyst, thus promoting reactions in our system (Small 2023, 16, 2301280;Energy Environ. Sci. 2023, 16, 3462-3473).
Based on previous reports, we have conducted the following experiments to demonstrate the existence and importance of the LSPR effect in this system: 1.The results of UV-vis-NIR absorption spectra show that Mo2N/MoO2-x nanosheets exhibit a significant absorption enhancement in the vis-NIR region.The plasmonic peaks, affected by varying catalyst grain sizes (Fig. 2a and Supplementary.Fig. 2), would overlap and cover each other, resulting a broad absorption range.Similar phenomena can also be observed in TiN (Nano Energy, 2022, 104, 107989).
2. We have found that the surface temperatures and CO generation rates of MNO-550 are promoted with increasing light intensity (Supplementary.Fig. 12b).The remarkable distinction of surface temperature indicates that the hot electrons from LSPR effect can accumulate on the catalyst for thermal energy generation, which presents an excellent photothermal conversion capacity of Mo2N/MoO2-x nanosheets (Adv.Mater. 2022, 34, 2202367).
3. The finite-difference time-domain (FDTD) simulations can also confirm the existence of strong LSPR effect by the local electric field enhancement at the interface of Mo2N and MoO2-x (Fig. 2b).

The authors emphasize that the synergy between Mo2N and MoO2-x is crucial to
optimize the RWGS reaction activity.However, it seems that Mo2N and MoOx just serve as separate sites to activate H2 and CO2, respectively.

Reply:
We appreciate the questions raised by the reviewer.The individual activities of Mo2N and MoO2-x are not high as shown in Fig. 2c.Based on the reviewer's suggestions, we mix Mo2N and MoO2-x powders physically and mechanically in different proportions and then test their photothermal CO2 hydrogenation performance.As shown in Fig. R7 (Supplementary Fig. 7), the activities of all mixed samples are below 3 mmol•gcat -1 •h -1 , much lower than the activity of MNO-550 (385 mmol•gcat -1 •h -1 ).
Moreover, our theoretical calculation shows that the free energy of *COOH dehydroxylation to form *CO on Mo2N/MoO2-x is much lower than that on Mo2N and MoO2-x surface.The comprehensive characterizations and theoretical calculations in our manuscript demonstrate that H2 and CO2 can be adsorbed and activated on the N sites and O vacancies simultaneously.Efficient CO2 hydrogenation demands the coordinating activation of H2 and CO2 (Nat. Commun. 2021, 12, 3884;ACS Energy Lett. 2021, 6, 2024-2029).In our work, we have achieved the outstanding photothermal RWGS performance results from the synergistic effect of uniformly distributed Mo2N and MoO2-x in the catalyst.Combining previous literature and our experimental and theoretical results, we can conclude that Mo2N and MoO2-x do not just serve as active sites for H2 and CO2 activation separately.By regulating the interaction between two components, we could enhance the synergistic effect of Mo2N and MoO2-x through a facile one-step annealing method, fully utilizing their respective advantages, which simultaneously achieve high activity, high selectivity, and long-term stability.7.Although the oxygen vacancy concentrations of 550,and 650 are very different, their ability to adsorb CO2 is similar (Supplementary Fig. 14).Therefore, the relationship of oxygen vacancies and CO2 adsorption is ambiguous in this manuscript.
Reply: We appreciate the reviewer's valuable comments.The difference between CO2-TPD peak of our samples is indeed not significant enough.The intensities of peaks at 531.5 eV can be attributed to Vo as shown in the O 1s XPS spectra of different samples in Fig. 3b.It is not difficult to comprehend that the concentration of oxygen vacancies is determined by the annealing temperature as higher annealing temperature leads to a more sufficient ammonization reaction and fewer oxygen vacancies.In order to seek more convincing evidence of the relationship between oxygen vacancies and CO2 adsorption, we have performed NAP-XPS to examine the adsorption of CO2 on different MNO samples in the dark.Upon feeding CO2, the O 1s XPS peaks results are shown as Fig. R8.The higher binding energy in MoOx (brown) signifies a more vital interfacial interaction among the nano-architecture with rising annealing temperatures.
We can distinctly observe that the peak intensity of adsorbed CO2 (magenta), oxygen vacancy (blue) and metal oxide (brown) all decrease monotonously with increasing annealing temperatures.We can introduce more oxygen vacancies and strengthen CO2 adsorption by raising the ammonization annealing temperature.According to previous research on MoO2 for solar-driven CO2 conversion (Angew.Chem. Int. Ed. 2023, 62, e202213124;J. Mater. Chem. A 2021, 9, 13898), the adsorption of CO2 highly depends on the oxygen vacancy.Therefore, combined with the prior results, these findings allow us to conclude that CO2 adsorption capacity is contingent upon oxygen vacancy concentration in our samples.Reply: We thank the reviewer for his/her valuable comments.These two works can indeed represent noble metal/metal oxide plasmonic nanostructures for CO2 conversion.
In the first article (Catal. Sci. Technol. 2015, 5, 2590-2601), the authors reported that an oxide-supported Au catalysts showed 30 to 1300% higher activity for RWGS under visible light compared to dark conditions.Their kinetic results indicated that LSPR effect increases the rate of hydroxyl hydrogenation and carboxyl decomposition.In the second work (ChemCatChem 2021, 13, 4507-4513), the authors demonstrated that Au/TiO2 is able to selectively promote the RWGS reaction under slightly concentrated artificial sunlight illumination without any external heating source.Moreover, they found that light helps to efficiently promote CO formation and suppresses the methanation reaction.The above two works both illustrate the advantages of photothermal catalysis in RWGS and the contribution of LSPR in the reaction.We have added relevant literature in the revised manuscript: "For example, Au/TiO2 is one typical photocatalyst with LSPR effect.Reply: We appreciate the reviewer's valuable comments.We agree with the reviewer's comments that H2 dominates the industrial cost of CO2 hydrogenation.However, we still believe that developing efficient catalysts and reducing the use of noble metals are extremely necessary.Moreover, it is significative to conduct an in-depth investigation of the LSPR effect on a non-noble metal catalyst.We have revised the manuscript as follows: "However, these catalysts still require the involvement of noble metals to activate H2 and boost their catalytic activity, which increases the intricacy of the catalyst synthesis process".

You tested the long term stability (160 h) of you catalyst under steady state operating
conditions (Fig. 2d).Normally, one would expect a slight decrease of the performance over time due to water adsorption.You don't seem to see that.Why is that the case?
What are the catalyst bed temperature and reactor temperature?
Reply: We thank the reviewer for his/her constructive suggestions.We have extended    Reply: We thank the reviewer for his/her valuable comments.Due to the various reaction conditions used by different research groups in photothermal catalysis, it is difficult to compare the activity under identical conditions.In order to increase the clarity and reliability of activity comparison, we have summarized and listed the specific testing conditions for the photocatalytic activity in different works in SI, shown as Table R3 (Supplementary Table 2).Our catalyst activity is still leading among these catalysts under the same lighting and atmosphere conditions.Molina et al. reported that the Au/TiO2 activity is 13.4 mmol•gcat -1 •h -1 at 1.4 W•cm -2 , and our MNO-550's activity can reach 18 mmol•gcat -1 •h -1 under 1 W•cm -2 light intensity, which is still slightly higher than their experimental results.

Fig
Fig. R1 and revised Supplementary Fig. 5.The production rate and selectivity of CO evolution for photothermal catalytic RWGS reaction in a batch reactor (a) by MNO-550 at different reaction times and (b) by Mo2N/MoO2-x with different annealing temperature under 3 W•cm −2 light illumination for 5 min.

2.
Fig. R2 and revised Supplementary Fig. 17.The KIE results for thermal catalysis and photothermal catalysis.

Fig. R3 and
Fig. R3 and revised Supplementary Fig. 6.The CO yield rate and selectivity of MNO-550 under the excited light with different wavebands for 5 min.3. We have used the different wavebands of light to stimulate reactions to comprehend the function of light (a light density of 2 W•cm −2 ).The CO yield rate of MNO-550 positively correlates with the absorption spectra (Fig. R3/Supplementary Fig.6).Due to the superimposed plasmonic characters of MoO2-x and Mo2N in the visible light region, the CO yield rate can reach up to 1180 mmol•gcat -1 •h -1 for 5 min under visible light excitation.Because of the insufficient energy density, almost no products can be detected in such a system under single wavelength light illumination.Therefore, we have provided the Light Energy to Chemical Energy Conversion Efficiency (LTC) and Thermal Energy to Chemical Energy Conversion Efficiency (TTC),referring to the calculation method in the literature(Ref S1.Nat.Common.2023, 14,

Fig
Fig. R4 and revised Supplementary Fig. 10.The CO generation rate and selectivity of MNO-550 under long-term alternating on/off light conditions (3W•cm −2 ).
. The comprehensive characterizations (FT-EXAFS spectra, XANES spectra, in-situ DRIFTS spectra, In-situ NAP-XPS spectra, KIE test, TPD test, photoelectrochemical test, TOF-SIMS spectra, EPR spectra, and others) and theoretical calculations (FDTD, DFT) help us to in-depth understand the catalyst and the reaction process, such as the electronic and morphology structural information, plasmonic photothermal effect and the corresponding activation process of reactants.In such a unique nano-architecture, H2 and CO2 can be adsorbed and activated on the N sites and O vacancies simultaneously.The synergistic effect of these active sites plays a crucial role in a significant reduction of the reaction energy barrier.Meanwhile, the existence of LSPR effect in the noble-metal-free catalyst efficiently enhances the conversion of photon energy to thermal energy and optimizes the localized energy regulation, further promoting the activation of reactant molecules and facilitating the reaction.This work employs a practicable design strategy to establish tunable synergistic sites with LSPR effect and provides methodological support for understanding the mechanism of photothermal CO2 hydrogenation.Reviewer 2In this manuscript, Wan et al. have designed an efficient photothermal catalyst ofMo2N/MoO2-x, which exhibits a combination of high activity and stability to catalyze the RWGS reaction under relative mild reaction conditions.The presence of localized surface plasmon resonance (LSPR) effect efficiently converts photon energy into localized heat, thus promoting the activation of H2 and CO2 at N atoms and O vacancies, respectively, on the catalyst surface.This work can be regarded as a good reference to guide advanced catalysts in photothermal catalysis by utilizing the LSPR effect.However, although various in-situ characterization techniques have been measured to investigate the structure-function relationship, the intrinsic structure of the catalyst and the origin of the LSPR effect are ambiguous, which largely limits the innovation and impact of this work.In summary, this manuscript needs major revisions.
On the other hand, the low NH3 flow rate (<10 SCCM) and low heating rate (1 ℃/min) lead to the slow ammoniation of MoO3, and the O atoms in the lattice are gradually replaced by N atoms.The formation of Mo2N is limited by growth kinetics and annealing temperature, and consequently, MoO3 cannot be completely ammoniated toMo2N (J.Phys.Chem.C 2010, 114, 14710-14715).4.The authors mention that the composition ratio ofMoO2-x and Mo2N can be tuned by controlling the annealing temperature.Is it possible to quantify the composition ratio of Mo2N and MoO2 accurately?Reply: We thank the reviewer for his/her valuable question.N and O are two kinds of light atoms with close atomic radii while Mo has abundant valence states, which make it hard to distinguish MoO2-x and Mo2N for accurate quantification.XPS, as a semi-quantitative characterization technique, can partially reflect the variation pattern of different component ratios.Here, we have performed peak fitting on Mo 3p XPS spectra of different samples, shown as Fig. R6 and Table R2.Each sample contains an evident MoO3 substrate phase.The ratio of Mo2N to MoO2 increases from 0.22 (MNO-450) to 0.68 (MNO-650) with the increasing ammoniation temperature, in line with expectations.

Fig.
Fig. R6 and revised Fig. 1f.Mo 3p and N 1s XPS spectra of Mo2N, MoO2-x and Mo2N/MoO2-x samples at different annealing temperatures.The orange, blue, green and magenta peaks are attributed to MoO3, MoO2, Mo 3p and N 1s of Mo2N components, respectively.

Fig.
Fig. R7 and revised Supplementary Fig. 7.The production rate and selectivity of CO evolution for physically mixed Mo2N and MoO2-x catalysts under 3 W•cm -2 fullspectrum light irradiation.Data of MNO-550 are also listed for comparison.

Fig. R8 and
Fig. R8 and revised Supplementary Fig. 15.In-situ O 1s NAP-XPS spectra for various samples in 0.3 mbar CO2 without illumination.The magenta, blue, and brown peaks are associated with oxygen originating from adsorbed CO2, oxygen vacancy and metal oxide, respectively.
the stability testing time to 190 hours, and a slight decrease of the performance can be observed at 190 th hour (12%).The stability results have been updated in the revised manuscript as Fig.2d.During the reaction process, the surface temperature of the catalyst can reach 250 ℃, while the temperature measured from the bottom of the reactor is about 65 ℃.

Fig.
Fig. R9 and revised Fig. 2d.Long-term stability test of MNO-550 in a flow reactor.

Fig.
Fig. R10 and revised Supplementary Fig. 10.Long-term stability test of MNO-550 for repetitive on/off experiment.

Table R2 .
The ratio of Mo2N to MoO2 calculated by peak fitting of Mo 3p.
8. Generally, metal sites have a strong capacity to dissociate H2 molecule.ForMo2N/MoOx, how to understand the role of exposed Mo sites in dissociating H2?
1.For noble metal/metal oxide combinations, you report systems with rather low activity (lines 61-67).One of the higher activities reported to date are obtained for Au/TiO2.See e.g.Plasmon-enhanced reverse water gas shift reaction over oxide supported Au catalysts -Catalysis Science & Technology (RSC Publishing) and Low Temperature Sunlight-Powered Reduction of CO2 to CO Using a Plasmonic Au/TiO2 Nanocatalyst -Martínez Molina -2021 -ChemCatChem -Wiley Online Library.
18-20 Fan et al.proved that hot electrons generated by LSPR can promote the formation of oxygen vacancies in Au/TiO2 catalyst, facilitating the adsorption and activation of CO2. 19Sastre 5. Fig.2fimplies things that are not correct.It is misleading.You cannot simply compare the activity of your catalyst 1-to-1 to other reported catalysts, unless the test conditions are identical (e.g., same irradiance, same feedstock ratio, same flow, etc.).You results are e.g.generated with double the irradiance of the results reported byMolina et al. (see above).Furthermore, the fact that other groups report shorter stability may also simply be attributed to the fact that they didn't test it longer that a couple of hours.